On some new airplanes, the static stability of the plane has been purposefully relaxed to include instances of flight wherein the plane has neutral static stability. A significant benefit in fuel-efficiency, weight-savings, and drag-reduction can be realized with such a system, due to an allowable reduction in required tail size. An airplane with relaxed static stability may have some undesirable and unacceptable handling quality characteristics, though.
One characteristic is that a neutrally stable airplane will not naturally return to its previous airspeed after a change in pitch attitude. On a trimmed positive static stability airplane, the pitch attitude changes according to the pilot pushing forward or pulling back on the control column. When the column is released, the airplane will continue to fly at its new pitch for a short time. As the airspeed of the plane reduces, the airplane will have a natural tendency to nose up or nose down, returning to its previous trim speed. This tendency is an expected handling quality of conventional commercial aircraft, and one to which pilots virtually automatically respond in flying their craft.
Neutrally stable planes have no tendency to return to their trim position and airspeed. after an adjustment of pitch. For these aircraft, changes in pitch are maintained and airspeed varies accordingly. During pitch increases, airspeed decreases. During pitch decreases, airspeed increases. To return the aircraft to its trim position and speed, the pilot must return the plane to its trim pitch, by repositioning the elevators with the control column.
A characteristic that is common to planes with or without relaxed static stability is that the amount of elevator input required to accomplish a certain pitch is sensitive to changes in weight and location of center of gravity relative to the mean aerodynamic center. An aircraft that is light in weight and has a center of gravity near the mean aerodynamic center, requires only a small change in elevator angle of attack to produce a large change in airplane pitch. Conversely, a heavy aircraft having a forward center of gravity requires much more elevator deflection to produce a like change in airplane pitch. Without control augmentation, the pilot must move the control column much farther in the heavy case in order to get the same pitch response as in the light case. This can be an undesirable characteristic because it requires the pilot to adjust his or her column input based on the weight and location of the plane's center of gravity--pieces of information not readily perceptible.
A third characteristic, though not related to relaxed stability aircraft, concerns the manner in which pilots expect the airplane to respond when the control column is moved. It is fairly well known that at low airspeeds, pilots expect movement of the control column to produce a change in pitch rate. At high airspeeds, pilots expect movement of the control column to produce a change in normal acceleration. This situation was recognized in the 1960's and the C* criterion was developed as a way to express optimal airplane response taking it into consideration. The C* criterion is discussed in detail below.
Thus, there exists a need for a superior pitch attitude control system capable of improving the control and handling characteristics of an aircraft. Optimally, this pitch attitude control system should provide such an airplane with handling characteristics (from the pilot's point of view) similar to an ideal conventional airplane with positive static stability, in order to reduce the amount of variation in flying technique required. The ideal control system should accomplish this goal by meeting the pilot's expectation regarding the airplanes short and long term responses to elevator command; and by responding to pilot pitch commands in a similar manner, regardless of weight or center of gravity shifts. The ideal control system should further accommodate the pilot's expectations to control pitch rate during low airspeed maneuvers and control to normal acceleration during high airspeed maneuvers. As will be appreciated by the following description, the present invention is directed to providing such a superior pitch attitude control system.